I note that Diogenes documents both dates (the 7th and the 10th) within one page of each other, the first one being a citation to Apollodorus, and the second being a citation of Epicurus' Last Will: “He was born, according to Apollodorus in his Chronicles, in the third year of the 109th Olympiad, during the archonship of Sosigenes, on the seventh day of the month of Gamelion, seven years after the death of Plato" (498). And then, one page later, "[T]he customary celebration of my birthday each year on the tenth of Gamelion.” (Mensch, Lives of Eminent Philosophers by Diogenes Laertius 499)
Since Both of these citations come from Diogenes, and they occur within a few paragraphs of each other, I think it is likely that the tenth is a symbolic custom, designated by Epicurus, and the seventh is the actual date of his birth.
The only proposition I have heard that corresponds with our Calendar comes from Alpers (about whom I could not find more information besides a citation from Alger) who proposes that the 24th of January corresponds with the 20th of Gamelion. So, if we are to take the 7th of Gamelion to be the actual date of Epicurus’ birth, then that should work out to January 11, 341 BCE. And then, the "customary celebration" of Epicurus' birth would fall on January 14, each year.
From my sources, I am drawing the following conclusions:
The 7th of Gamelion is the actual date of Epicurus' birth (Jan. 11th?)
The 10th of Gamelion is the date of celebration for those who strictly follow Epicurus' Last Will (Jan. 14th?)
The 20th of Gamelion is the date of celebration for the Eikadistae who merged his Birthday with Gamelion Eikas (Jan. 24th?)